Method and apparatus for automatic relative placement generation for clock trees

ABSTRACT

Methods and apparatuses are disclosed for automatic relative placement of part of a clock tree in the course of generating a placed, routed, and optimized circuit design.

PRIORITY APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/659,126 filed on 16 Mar. 2015 (now U.S. Pat. No. 9,430,601), whichapplication is a continuation of U.S. patent application Ser. No.13/212,061 filed on 17 Aug. 2011, now U.S. Pat. No. 8,984,467, whichapplications are incorporated herein by reference.

RELATED APPLICATION

This application is related to U.S. Pat. Nos. 7,581,197 and 7,937,682,which are incorporated herein by reference.

BACKGROUND

Field

The technology relates to integrated circuit fabrication, and moreparticularly to placement, routing, and optimization of an integratedcircuit design that obeys rules that specify the relative placement ofcircuit elements.

Description of Related Art

An integrated circuit design flow typically proceeds through thefollowing stages: product idea, EDA software, tapeout, fabricationequipment, packing/assembly, and chips. The EDA software stage includesthe steps shown in the following table:

EDA step What Happens System Design Describe the functionality toimplement What-if planning Hardware/software architecture partitioningLogic Design and Write VHDL/Verilog for modules in system FunctionalCheck design for functional accuracy, does the Verification designproduce correct outputs? Synthesis and Translate VHDL/Verilog to netlistDesign for Test Optimize netlist for target technology Design andimplement tests to permit checking of the finished chip Design PlanningConstruct overall floor plan for the chip Analyze same, timing checksfor top-level routing Netlist Verification Check netlist for compliancewith timing constraints and the VHDL/Verilog Physical Placement(positioning circuit elements) and Implement. routing (connectingcircuit elements) Analysis and Verify circuit function at transistorlevel, allows Extraction for what-if refinement Physical Variouschecking functions: manufact., electrical, Verfication (DRC,lithographic, circuit correctness LRC, LVS) Resolution Geometricmanipulations to improve Enhanc. (OPC, manufacturability PSM, Assists)Mask Data “Tape-out” of data for production of masks for Preparationlithographic use produce finished chips

With regard to physical implementation technology, methodologies forstructured placement offer circuit designers superior power, yield,and/or area for a given logic function. With the advent of manualplacement of transistors, designers created arrayed layouts where logicgates were manually placed in a regularized fashion. This methodologyhas evolved to the point that automation has been applied to theproblem. However, regularized placement still suffers from a great dealof manual effort, such as in cell drive strength selection.

Capturing a priori designer knowledge of structured placementrequirements in HDL is a nontrivial problem. Even if captured,structured placement requirements are lost during standard cell randomplacement. Standard cell placers tend to take more localized viewsduring optimization. This results in not just loss of regularity, butalso extra buffering, routing, vias, and cell oversizing, compared to asolution which might be obtained following structured placement.

One approach to this problem is to perform cell sizing and optimizationof a structured placement manually through homegrown tools. Thisapproach is quite expensive in terms of engineering effort. Thisapproach is also hard to integrate with the rest of the design. Suchintegration requires multiple iterations, because standard cellplacement and optimization changes the placement, sizing, etc. of thesurrounding, non-structured logic. Unfortunately, this triggers anotheriteration, with further manual sizing and optimization efforts throughthe homegrown tools for the block with structured placement.

Other approaches to this problem are to generate structured placementthrough synthesis or through a specific tool, and then pass on theresult to a placer through a set of special constraints, or as amacro/IP block. The initial structure generated through synthesis orthrough the special tool is problematic. Because the initial structureis generated prior to placement, the initial structure is formed lackingcomplete placement knowledge, and thus the initial structure fails tolead to optimal placement as generation. Also if it is passed as amacro/IP block, then place and route tools cannot resize or otherwiseoptimize the blocks.

Relative placement rules as applied to clock trees are problematic.Several of the disadvantages follow. Relative placement rules specifiedin a scripting language such TCL can require updates as the relativeplacement language changes. The design can be over-constrained and moreclock gates inserted than needed, and clock gate incorrectly sized. Amanual iterative process of figuring the maximum distance of theflip-flops to cluster is labor intensive. There can be inability tocluster and create smaller relative placement groups when a clock gatedrives more than the maximum fanout limit of a clock gate. It is laborintensive to manually create relative placement rules and manuallymodify the netlist (connect/disconnect gates/flops) and insert clockbuffers and create relative placement structures. Non-convergence can becaused by using ad-hoc/distance based techniques up front inplacement/optimization versus a native clock tree clustering algorithmused in clock tree synthesis, as they are not the same processes.

Therefore, it would be desirable to efficiently implement structuredplacement with circuit design.

SUMMARY

One aspect of the technology is a method of circuit design with acomputer system. The method includes generating with the computer systema placed, routed, and optimized circuit design. Generating the circuitdesign includes guiding coarse placement of the circuit design accordingto rules created specifically and automatically for a set of circuitelements in a netlist of the circuit design including clock networkflip-flops. The rules specify positioning of each circuit element of theset of circuit elements in the circuit design relative to other circuitelements of the set of circuit elements in the circuit design.Generating the circuit design also includes completing placement,routing, and optimization of the netlist of the circuit design accordingto the rules.

Various embodiments can include one or more of the following features.

The method further includes, after the coarse placement and before clocktree synthesis of the circuit design, automatically creating the clocknetwork flip-flops in the netlist of the circuit design.

The method further includes, after the coarse placement and before clocktree synthesis of the circuit design, automatically creating the clocknetwork flip-flops in the netlist of the circuit design based on clocksinks in the netlist.

The method further includes, after the coarse placement and before clocktree synthesis of the circuit design, automatically grouping the clocknetwork flip-flops into a plurality of flip-flop groups.

The method further includes, after the coarse placement and before clocktree synthesis of the circuit design, automatically grouping the clocknetwork flip-flops into a plurality of flip-flop groups. The rulesautomatically created for the clock network flip-flops are based on theplurality of flip-flop groups.

The method further includes, after the coarse placement and before clocktree synthesis of the circuit design, performing clock tree clusteringthat automatically groups the clock network flip-flops into a pluralityof flip-flop groups.

The method further includes, after the coarse placement and before clocktree synthesis of the circuit design, performing clock tree clusteringthat automatically groups the clock network flip-flops into a pluralityof flip-flop groups, without creating clock network buffer circuitry.

The method further includes, after the coarse placement and before clocktree synthesis of the circuit design, performing clock tree clusteringthat automatically groups the clock network flip-flops into a pluralityof flip-flop groups. The rules automatically created for the clocknetwork flip-flops are based on the plurality of flip-flop groups fromthe clock tree clustering.

The method further includes, after the coarse placement and before clocktree synthesis of the circuit design, performing clock tree clusteringthat automatically groups the clock network flip-flops into a pluralityof flip-flop groups. The method further includes, during placement ofthe circuit design, automatically creating clock network buffercircuitry for the plurality of flip-flop groups.

In the method, the clock network flip-flops are leaf nodes of a clocktree of the circuit design.

The method further includes, during placement of the circuit design,automatically adding clock network buffer circuitry matched to thegrouped clock network flip-flops in the circuit design.

In the method, the rules obey clock tree constraints specified for thecircuit design.

Other aspects of the technology can be embodied in a correspondingsystem, apparatus, and computer readable medium.

The data processing system is adapted to process a computer implementedrepresentation of a circuit design. The system has a data processor andmemory coupled to the data processor. The memory stores instructionsexecutable by the data processor. The instructions perform a process asdescribed herein.

The apparatus can be a circuit. The circuit follows a placed, routed,and optimized circuit design. The placed, routed, and optimized circuitdesign is created by a process as described herein.

The computer readable medium has computer readable instructionsexecutable by a computer system. The instructions perform a process asdescribed herein.

Other embodiments include a computer readable medium with computerreadable instructions for circuit design, with computer instructionsthat perform the technology described herein; a circuit design generatedwith the technology described herein; and an electrical circuit withcircuitry designed by the technology described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating an exemplary process for usingrelative placement in circuit design.

FIG. 2 is a flow diagram illustrating an exemplary process of theplacement, routing, and optimization tool.

FIG. 3 shows the placement, as a single unit, of a group of circuitelements following relative placement rules.

FIG. 4 shows circuit elements organized by relative placement rules intocolumns and rows.

FIG. 5 shows the creation of a hierarchy of circuit elements organizedby relative placement rules.

FIG. 6 shows the creation of a multiple instances of circuit elementshierarchically organized by relative placement rules.

FIG. 7 shows an ungrouped version of the circuit design of FIG. 6.

FIG. 8 shows a circuit design with keepouts defined by the relativeplacement rules.

FIG. 9 shows cells that straddle multiple columns and multiple rows asdefined by the relative placement rules.

FIG. 10 shows the creation of a hierarchy of circuit elements organizedby relative placement rules that permit straddling of multiple rows andmultiple columns.

FIG. 11 shows a circuit design organized by relative placement ruleswith orientation optimization and pin alignment.

FIG. 12 shows a relative placement group of a circuit design aligned bypins.

FIG. 13 shows a relative placement group of a circuit design aligned bymultiple pins.

FIG. 14 shows a relative placement block of a circuit design thatcontains a group aligned by pins.

FIG. 15 shows a relative placement block of a circuit design anchored atthe x-coordinate and the y-coordinate.

FIG. 16 shows multiple relative placement blocks of a circuit designaligned and anchored vertically at four coordinates.

FIG. 17 shows circuit elements with and without compression specified byrelative placement rules.

FIG. 18 shows the placement of relative placement cells in a circuitdesign containing keepouts.

FIG. 19 is a simplified block diagram of a computer system suitable foruse with embodiments of the technology, as well as a circuit design andcircuit embodiments of the technology.

FIG. 20 is a flow diagram illustrating an exemplary process of theplacement, routing, and optimization tool with clock tree synthesisclustering being performed during placement.

FIG. 21 is a flow diagram illustrating another exemplary process of theplacement, routing, and optimization tool with clock tree synthesisclustering being performed during placement.

DETAILED DESCRIPTION

Relative placement information generates a structure of the instancesand controls the placement of the instances. The resulting annotatednetlist is used for physical optimization, during which the placement,routing, and optimization tool preserves the structure.

FIG. 1 shows the overall flow for using relative placement. Relativeplacement usually is applied to datapaths and registers, but relativeplacement can be applied to any cells in a circuit design, controllingthe exact relative placement topology of gate-level logic groups anddefining the circuit layout.

At 110, a mapped netlist (a structural interconnection of library cells)is read. At 140, a netlist annotator adds annotations of the relativeplacement constraints to the mapped netlist. The relative placementconstraints may have come from an automatic rule creator 120, whichgenerates automatically created relative placement rules 125. Also, therelative placement constraints may have come from the circuitdesigner/custom tool 130, which generates relative placement rulescreated by the circuit designer 135. Based on the mapped netlist 110 andthe relative placement rules, the netlist annotator 140 generates anetlist annotated with relative placement rules 145.

The netlist annotator 140, which may be GUI-based or text-based providesa way create relative placement structures for the placement, routing,and optimization tool 150. In a GUI-based annotator, the relativeplacement can be specified by drag-and-drop of circuit elements intopositions relative to other circuit elements. Clicking multiple circuitelements and assigning an identifier such as a color, a pattern, or aname can define multiple groups with respective relative placementrules. With a text-based annotator, relative column and row positionscan be specified of instances with respect to each other. Theseplacement constraints create relative placement structures that arepreserved during placement and legalization. Whether GUI-based ortext-based, the cells in each structure group are placed as a singleentity.

The placement, routing, and optimization tool 150 receives the netlistannotated with relative placement rules 145 and generates the placed,routed, and optimized netlist obeying relative placement rules 155. Theoptimization includes, for example, orientation optimization.

FIG. 2 shows an exemplary process flow of a placement, routing, andoptimization tool. At 201, an annotated netlist is preprocessed for therelative placement annotation. Data structures are created to carryrelative placement information. At 202, the sizes of relative placementblocks and aspect ratios are estimated by applying relative placementrules for each relative placement block. Any hierarchical relativeplacement blocks are estimated also. At 204, each of the estimatedrelative placement blocks is modeled for coarse placement purposes, forexample as a macro with pin locations visible but the internal cellshidden from the coarse placer. At 206, the relative placement blocks areplaced within the context of entire design, including the cells whichhave the relative placement rules simultaneously along with the cellswhich do not have relative placement rules. Among the relative placementblocks of cells, the blocks are placed one at a time. At 208, if neededanother incremental placement is done for area recovery of sparserelative placement blocks. One block at a time is fixed based on thelocations returned by the coarse placer. Individual relative placementinstances are fixed before such area recovery. At 210, individualinstances of cells are readjusted within each relative placement block,based on new locations determined by the placer according to theoptimization constraints. User constraints are respected for eachrelative placement block. At 212, the nearest legal locations for allthe relative placement cells are found, and the relative placement cellsfixed there. Any overlaps of relative placement blocks are resolved bychecking each of the already fixed blocks and making sure that the movedblocks do not overlap with them. If overlaps occurs, the moved blocksare moved with minimal movement as the cost. At 214, detailed placementis done for the non-relative placement cells, considering the fact thatcells with relative placement are already fixed. At 216, all relativeplacement cells are unfixed. If optimization and relative placementconstraints are met, then the tool can stop, finish writing out theplaced and routed netlist, and exit the placement and optimizationprocess. At 218, physical optimization is done for all the instances,including relative placement instances, to meet timing or any other userspecified goals. This could focus on the most critical objectives suchas timing, respecting design rules, congestion, wire length etc. Theoptimization includes, for example, orientation optimization. At 220,relative placement constraints are reapplied, and locations readjustedbased on optimization results. Thus, the relative placement constraintsspecified in the annotated netlist are preserved. The process then loopsback to 212.

The above process can be rearranged, for example by combining, adding,removing, or modifying steps. For example, 212-216 can be rearranged,depending on the design, and 208 and 210 can be combined into one step.

Benefits of Relative Placement

Various embodiments that implement relative placement provide one ormore of the following benefits:

1) Provides a method to maintain structured placement for legacy orintellectual property (IP) designs using a placement, routing, andoptimization tool.

2) Handles flat and hierarchical designs.

3) For complex designs, a typical design can have many engineers workingon it and many blocks. Hierarchical relative placement enables placingthose blocks together relative to each other more easily. Any number oflevels of hierarchies are allowed.

4) Reduces the placement search space in critical areas of the designresulting in greater predictability of QoR (wire length, timing, power)and congestion.

5) Is technology independent.

6) Improves routability.

Relative Placement Considerations

Various embodiments that implement relative placement require one ormore of the following considerations:

1) When the placement, routing, and optimization tool estimates that thesize of a relative placement block is not suitable to the givenfloorplan, the placement, routing, and optimization tool can fail inplacement. To maintain relative placement information precisely, thereshould be enough space for relative placement blocks without overlappingplacement obstructions in the design floorplan.

2) If the design contains multiheight cells and exact relative placement(perfect alignment of the cells on one or more sides of the row orcolumn) is used, the current relative placement implementation might notget perfect alignment in every case.

3) There is no limit on the number of cells in a relative placementgroup. However, if the design has many relative placement groups, attimes coarse placement returns overlapping group locations, resulting inmisalignment. In these cases, a warning appears after coarse placement.

The following is a specific exemplary implementation of the discussedprocess flow. Many of the examples which follow are implemented with atext-based shell. The text-based examples are provided for in anexemplary Synopsys™ design environment for the purposes of illustration.The examples are also applicable to a GUI-based environment, in whichthe text-based commands are replaced or complemented with a mouse-driveninterface.

Exemplary Relative Placement Flow

Implementing the methodology for the relative placement flow followsseveral steps.

1. In a design environment that permits a user to decide whether or notto use relative placement, relative placement is enabled. Relativeplacement is enabled by entering “XG mode”, performed by entering theshell command: psyn_shell-xg-t>set physopt_enable_rp_in_xg_mode “true”

2. The gate-level netlist is prepared and read it in to the placement,routing, and optimization tool, using the read_milkyway or read_dbcommand.

3. The relative placement data are prepared.

Create the relative placement groups. Use the create_rp_group command.

Add relative placement items to the groups. Use the add_to_rp_groupcommand.

The netlist annotator annotates the netlist with the relative placementinformation, and generates a placed netlist containing the data.

4. Preserve the relative placement information in the annotated netlist.Use set_size_only to preserve relative placement information for cellsthat contain it. For example, enter

-   -   psyn_shell-xg-t>set_size_only {RP_cells} true

5. Set the block utilization of the relative placement block. The blockutilization is how densely a block is packed. A value of 1 indicates nogap between columns. It could vary between 0 and less than or equalto 1. Enter

-   -   psyn_shell-xg-t>set physopt_use_block_utilization true

The default value is 1.0.

6. Read floorplan information. For example, enter

-   -   psyn_shell-xg-t>read_pdef top.pdef

7. Perform coarse placement for the design. Use the create_placementcommand.

8. Analyze the design using the placement, routing, and optimizationtool GUI.

9. If the relative placement result is not acceptable, modify therelative placement file and run this procedure again.

If the relative placement is acceptable, then perform optimization, byrunning physopt.

Sample Script for a Relative Placement Flow

The following is a sample script for running a relative placement flow.

 # Set library and design paths #  source setup.tcl  setphysopt_enable_rp_in_xg_mode ″true″  #Read db file generated fromrp_reader #  read_db block_rp.db  read_db block2_rp.db  read_dbother_blocks.db  read_db top.db  current_design top  link link_physical_library  # Create relative placement #  create_rp_group ...  add_to_rp_group  ...  # Apply constraints #  sourceconstraints.tcl  # Apply set_size_only on relative placement cells topreserve relative placement information #  set_size_only {RP_cells} true # Read floorplan information #  read_pdef top.pdef  # Perform coarseplacement on the design #  create_placement  # Perform analysis. Ifokay, do physical synthesis or #  # incremental physical synthesis # physopt or physopt -incremental

Considerations for Using Relative Placement

A design can contain both structured and unstructured items (leaf cells,keepouts, hierarchical groups). Control of which cells are to bestructured is accomplished by including the cells to be structured in arelative placement group.

Determining which portions of the module need to be structured isbeneficial. Providing relative placement information for cells thatwould have been placed better by allowing the placement, routing, andoptimization tool to place the cells can produce poor results.

Some designs are appropriate for structured placement (for example,datapaths), whereas others are more appropriate for usual placement bythe placement, routing, and optimization tool.

Data Required for Relative Placement

Relative placement requires a gate-level netlist. The format can be anyformat read by the placement, routing, and optimization tool.

Commands for Relative Placement

The basic functionality for relative placement is carried out by way ofdedicated Tcl commands used within the placement, routing, andoptimization tool. The commands create groups and add leaf cells,hierarchy, and keepouts to the groups. In addition, a script ofannotated information can be generated, edited, and reapplied to thedesign, and relative placement groups can be removed.

In addition to these dedicated commands, physical synthesis commands areavailable.

Commands Specific to Relative Placement

Relative placement uses the dedicated commands listed in Table 1.

TABLE 1 Dedicated Commands for Relative Placement Command Described insection create_rp_group See “Creating Relative Placement Groups”.add_to_rp_group See “Adding Items to a Group”. write_rp_group See“Writing Relative Placement Information to a Script File”.remove_rp_group See “Removing Relative Placement Information”.

Other Commands Often Used for Relative Placement

The commands listed in Table 2 are often used for relative placement.

TABLE 2 Other Commands Often Used for Relative Placement CommandDescribed in section set_size_only See “Preserving Relative PlacementInformation During Optimization”. create_bounds See “ConstrainingRelative Placement Cell Placement Using Placement Bounds”. find Locatesand returns a collection of relative placement groups that match listednames. The man page has information. source Loads and runs a scriptfile. The man page has information.

Preserving Relative Placement Information During Optimization

The relative placement information for an instance is attached to theinstance. During optimization, relative placement cells can be optimizedor removed. When an instance with relative placement information isremoved during optimization, relative placement information attached tothe instance is also removed.

To prevent relative placement cells from being removed duringoptimization, apply set_size_only to true on leaf cells to preserverelative placement information for cells that contain it. If relativeplacement cells are upsized or downsized, the relative placement cellalignment is maintained for placement.

Constraining Relative Placement Cell Placement Using Placement Bounds

The placement of relative placement cells is constrained by definingplacement bounds. To do this, use the create_bounds command. Both softbounds and hard bounds are supported for relative placement cells andboth rectangular bounds and rectilinear bounds are supported.

Note: In relative placement, only move bounds (with fixed coordinates)are supported for relative placement cells. Group bounds are notsupported. In other embodiments, group bounds are supported in relativeplacement.

Specify individual cell names as provided in an add_to_rp_group commandwith the create_bounds command. For example, enter

-   -   psyn_shell-xg-t>create_bounds -coordinates {100 100 200 200} U1        U2 U3 U4

In other embodiments, a relative placement group is specified.

If some cells of a relative placement group are specified to be inside abound and some cells are not specified to be inside the bound, cellsthat are not constrained by the bound are placed as loose cells. Thiscan lead to legally correct but possibly poor placement QoR.

Ignoring Relative Placement Information

The tool can be directed to ignore relative placement informationannotated to the design, for example, when to confirm that relativeplacement is helpful to QoR. To do this, set the variablephysopt_ignore_structure to true (default is false).

Setting this variable to true causes the placement, routing, andoptimization tool not to do structured placement for any relativeplacement groups in the design.

When the tool is directed to ignore relative placement information, theparts of the relative placement groups are placed as if the group has norelative placement information.

Removing Relative Placement Information

Relative placement annotation can be removed from an annotated databasefor one or more relative placement groups. To do this, use theremove_rp_group command.

Note: When a relative placement group is removed, the memory it occupiesis freed to be reused by this same process. However, memory is notreturned to the operating system until exit from psyn_shell.

To remove relative placement groups, enter

-   -   psyn_shell-xg-t>remove_rp_group [options]

To do this Use this List the relative placement groups to remove. (vs.group_list using -all). Remove all relative placement groups. (vs. using-all -all). Remove all the designs within the hierarchies of -hierarchythe groups listed in the group list. By omitting this option, subgroupsare not removed. (vs. using -all). During removal, disable printing thegroups being -quiet removed.

Example

To remove the relative placement group named grp_ripple and confirm itsremoval, enter

psyn_shell-xg-t> find rp_group {mul::grp_mul ripple::grp_ripplexample3::top_group} psyn_shell-xg-t> remove_rp_group grp_ripple Removingrp group ’ripple::grp_ripple’ 1 psyn_shell-xg-t> find rp_groupgrp_ripple Error: Can't find object ’grp_ripple’. (UID-109)psyn_shell-xg-t> remove_rp_group -all Removing rp group ’mul::grp_mul’Removing rp group ’example3::top_group’ 1

Modifying Relative Placement Information

A relative placement group can be modified. For example:

-   -   Remove items from a group    -   Rename a group    -   Move an item from one position to another    -   Realign or reorient items

A group can be modified in the following ways:

-   -   Remove the group to be changed, then create a new group that        incorporates the changes.    -   Generate a script (using write_rp_group) and edit the        information in the generated script.

Creating Relative Placement Groups

A relative placement group is an association of cells, other groups, andkeepouts. During placement and legalization, the group structure ispreserved and the cells in the group are placed as a single entity. Tocreate a group, use the create_rp_group command. The group is placed asa single entity, as shown in FIG. 3. In FIG. 3, floorplan 302 is shown.The group 306 includes cells 310 and obstructions 312. The group can bemoved 308 as a single unit. The floorplan 302 also has RAM 304 which isfixed in place.

Positions for Columns and Rows in Relative Placement Data

FIG. 4 shows the positions for columns and rows in relative placementdata 400. Columns count from column 0 (the leftmost column). Rows countfrom row 0 (the bottom row). The width of a column is the width of thewidest cell in that column. The height of a row is determined by theheight of the tallest cell in that row.

In FIG. 4, positions 0 3 (column 0, row 3) and 4 1 (column 4, row 1) arenot used and therefore are not specified. Position 1 2 occupies(straddles) columns 1 and 2 in row 2. Position 4 3 in column 4 straddlesrows 3 and 4.

Straddling is described in “Creating Relative Placement StructuresContaining Multiple Column or Row Positions”.

The following points can apply to creating a new group:

-   -   A new group is empty. To add leaf cells, hierarchy (other        groups), and keepouts to the group, use the add_to_rp_group        command.    -   Any number of groups can be created.    -   Relative placement groups are persistently stored using the        write_milkyway command and read using the read_milkyway command.    -   The create_rp_group command returns a collection handle        (identifier) to the relative placement groups that are created.        If no objects were created, the empty string is returned.

To use the create_rp_group command, enter

-   -   psyn_shell-xg-t>create_rp_group [options]

To do this Use this Name the group group_name Specify the name of thedesign in which to -design create the new group. Omitting this option,defaults the design to the current design. Using this switch is goodpractice. Specify the number of columns for the -columns group,expressed as a positive integer. The default is 1. Specify the number ofrows for the group, -rows expressed as a positive integer. The defaultis 1. Specify the default pin name to look up on -pin_align_name a cell.By specifying -pin_align_name in an add_to_rp_group command, the valuespecified in add_to_rp_group overrides the value specified here. See“Aligning Relative Placement by Pins”. Use the group for hierarchyinstantiation. -instance See “Defining Hierarchical Groups forInstantiation”. Anchor the group on the x-axis or the -x_offset ory-axis but allow the group to slide in the -y_offset other dimension.The values are in microns and the offset is relative to the chip'sorigin. See “Anchoring Relative Placement Blocks at a SpecifiedLocation”. Anchor the group on both the x-axis and -x_offset and y-axisby specifying the lower-left -y_offset coordinates (anchor) for thegroup. The values are in micron and the offset is relative to the chip'sorigin. See “Anchoring Relative Placement Blocks at a SpecifiedLocation”.

Example

To create the group named rp1 for designA having 1 column and 3 rows,enter

-   -   psyn_shell-xg-t>create_rp_group rp1 -design designA -columns 1        -rows 3

Renaming a Group

A group cannot be renamed directly. To rename a group, remove the groupand create a new group that duplicates the removed group but has the newname. Alternatively, generate a script (using write_rp_group) and editthe name in the generated script. In other embodiments, the group can berenamed directly.

Adding Items to a Group

To add leaf cells, hierarchy groups, and keepouts to relative placementgroups (created using create_rp_group), use the add_to_rp_group command.

When adding an item to a relative placement group, the following pointscan apply:

-   -   The relative placement group in which the item is added must        exist. In another embodiment, a default group is created.    -   Switches identify whether the item added is a leaf cell (-leaf),        hierarchical group (-hierarchy), or a keepout (-keepout).

The syntaxes for adding leaf cells, hierarchy groups, and keepoutsdiffer. Table 3 provides a quick look up of the options allowed for eachsyntax.

-   -   If an item already exists in the group at the given column and        row location or if the item to be inserted is already        positioned, an error message appears.    -   The command returns a collection handle (identifier) to the        relative placement groups in which the objects are added. If no        objects are created, the empty string is returned.

Syntax for Adding a Leaf Cell

The syntax to add a leaf cell is

add_to_rp_group group_list -leaf cell_name [-column col_number] [-rowrow_number] [-pin_align_name pin_name] [-orientation direction]

Syntax for Adding a Hierarchical Group

The syntax to add a hierarchical group is

add_to_rp_group group_list -hierarchy group_name [-instanceinstance_name] [-column col_number] [-row row_number]

Syntax for Adding a Keepout

The syntax to add a keepout is

add_to_rp_group group_list -keepout keepout_name [-column column_number][-row row_number] [-width value] [-height value]

Options to Use to Add Items to Relative Placement Groups

Use appropriate options as shown previously in the syntaxes to add itemsto a relative placement group. The options used depend on the item to beadded to the group. Table 3 provides a quick look up for the optionsavailable for each add_to_rp_group syntax.

To do this Use this List the relative placement group names ingroup_list which to add items. The groups must be for the same design.In other embodiments, different designs are permitted. Specify thecolumn position in which to -column add the item (default is 0). Columnpositions start at 0, which is the leftmost column. Specify the rowposition in which to add -row the item (default is 0). Row positionsstart at 0, which is the bottom row. Add the named leaf cell. Each leafcell -leaf added must exist in the gate-level netlist. (vs. -hierarchyor -keepout for the position specified). In other embodiments, a defaultcell is created. Add the named relative placement group -hierarchy forhierarchy inclusion. The group can be used one time in another group inthe same design. See “Defining Hierarchical Groups for Inclusion”. (vs.-leaf or -keepout for the position specified). Specify the name of theinstance on which -hierarchy to instantiate the given hierarchicalrelative -instance placement group for hierarchy instantiation. A groupcan be instantiated more than once. See “Defining Hierarchical Groupsfor Instantiation”. Use -instance with -hierarchy. (vs. -leaf or-keepout for the position specified). Add the named hard keepout. Thereis no -keepout keepout object so the name provided here is to referencethe keepout after it is created. See “Adding Keepouts”. Specify the nameof the pin to use for pin -pin_align_name alignment of this cell withother cells in a group. If using -pin_align_name, the value specifiedhere overrides a pin name provided with create_rp_group for the relativeplacement group to which it is being added. See “Aligning RelativePlacement by Pins”. Specify the placement orientation of a cell-orientation with respect to the group to which it is added. PDEF or DEFsyntax can be used, as follows: PDEF orientations: 0, 90, 180, 270, 0-mirror, 90-mirror, 180-mirror, or 270- mirror DEF orientations: N, W, S,E, FN, FW, FS, or FE See “Specifying Orientation for Leaf Cells”.Specify the width of the keepout to add. If -width -width omitted, thekeepout defaults to the width of the widest cell in the column in whichthe keepout is added. Use this option with -keepout and -height. See“Adding Keepouts”. Specify the height of the keepout to add. If -height-height omitted, the keepout defaults to the height of the tallest cellin the column in which the keepout is added. Use this option with-keepout and -width. See “Adding Keepouts”.

Quick Lookup of Options for Add_to_rp_Group Syntaxes

Table 3 provides a quick lookup of the options available for theadd_to_rp_group syntaxes.

TABLE 3 Quick Lookup of Options for add_to_rp_group Syntaxes HierarchyHierarchy Leaf for for Option Syntax cell Inclusion instantiationKeepout group_list X X X X -column X X X X -row X X X X -leaf X-hierarchy X X -keepout X -pin_align_name X -orientation X -instance X-width X -height X

Example

To find relative placement group grp_ripple, add leaf cell U2 togrp_ripple, then instantiate grp_ripple in the top group, enter

psyn_shell-xg-t> find rp_group grp_ripple {ripple::grp_ripple}psyn_shell-xg-t> add_to_rp_group grp_ripple -leaf carry_in_1{ripple::grp_ripple} psyn_shell-xg-t> add_to_rp_group top_group-hierarchy grp_ripple -instance U2 {example3::top_group}

Adding Hierarchical Groups

Hierarchical relative placement allows relative placement groups to beembedded within other relative placement groups. The embedded groupsthen are handled similarly to leaf cells. To add hierarchical groups,use the add_to_rp_group command with its -hierarchy or -hierarchy and-instance switches, depending on the type of hierarchical group wanted.

Hierarchical relative placement simplified expression of relativeplacement constraints. With hierarchical relative placement, providingrelative placement information multiple times is unnecessary for arecurring pattern.

Benefits of Using Hierarchical Groups in Relative Placement

Various embodiments that implement hierarchical relative placementprovide one or more of the following benefits:

1) Allows organization of relative placement in a manner that is easierto maintain and understand. For example, the relative placement groupcan bed created to parallel Verilog or VHDL organization.

2) Allows reuse of a repeating placement pattern, for example, an adder.

3) Can reduce the number of lines of relative placement information tobe written.

4) Allows integrating blocks.

5) Provides flexibility for the wanted configuration.

Types of Hierarchical Relative Placement Group Usage

Hierarchical relative placement in different ways, depending on whetherthe relative placement group is used in the same design or in differentdesigns:

Inclusion

Applies to a relative placement group in the same design as the group inwhich it is included. An included group is used one time in the samedesign.

See “Defining Hierarchical Groups for Inclusion”.

Instantiation

Applies to a relative placement group that is not from the design inwhich it is instantiated. An instantiated relative placement group canbe used multiple times and in multiple places up to the number of timesthe design of the group is instantiated in the netlist.

See “Defining Hierarchical Groups for Instantiation”.

Important:

The syntaxes for creating the hierarchical group definitions forinclusion and for instantiation are the same except the use of -instanceswitch for instantiation.

Defining Hierarchical Groups for Inclusion

To specify that a group is a hierarchically included group, specifyhierarchy by using the -hierarchy switch with the add_to_rp_groupcommand.

When a group is included in a parent group, it is as if the group isdirectly embedded within the parent group. An included group can be usedin another group of the same design one time. However, the new groupthat contains the included group can be further included in anothergroup in the same design or instantiated in another group of a differentdesign.

See the syntax provided in “Syntax for Adding a Hierarchical Group” andthe options summary provided in Table 3.

Example

To include the relative placement group named rp3 as a hierarchicalgroup for inclusion in group rp4, enter

-   -   psyn_shell-xg-t>add_to_rp_group rp4 -hierarchy rp3 -column 0        -row 0

The script in the following example defines the input for a hierarchicalrelative placement definition for inclusion. Groups rp1, rp2, rp3, andrp4 are all defined as being part of design top (shown in bold). Thecontents of groups rp1, rp2, and rp3 are treated as leaf cells when theyare included in group rp4.

Example Hierarchical Relative Placement Definition for Inclusion

create_rp_group rp1 -design top -columns 2 -rows 1 add_to_rp_group rp1-leaf U1 -column 0 -row 0 add_to_rp_group rp1 -leaf U4 -column 1 -row 0create_rp_group rp2 -design top -columns 2 -rows 1 add_to_rp_group rp2-leaf U2 -column 0 -row 0 add_to_rp_group rp2 -leaf U5 -column 1 -row 0create_rp_group rp3 -design top -columns 2 -rows 1 add_to_rp_group rp3-leaf U3 -column 0 -row 0 add_to_rp_group rp3 -leaf U6 -column 1 -row 0create_rp_group rp4 -design top -columns 1 -rows 3 add_to_rp_group rp4-hier rp1 -column 0 -row 0 add_to_rp_group rp4 -hier rp2 -column 0 -row1 add_to_rp_group rp4 -hier rp3 -column 0 -row 2

In the above example,

-   -   Groups rp1, rp2, and rp3 are each defined as having two columns        and one row.    -   Group rp4, in which groups rp1, rp2, and rp3 are included (each        group used one time), is defined as having one column and three        rows.        -   Each included group is defined as a hierarchical subgroup            (group rp1 as subgroup rp1, group rp2 as subgroup rp2, and            group rp3 as subgroup rp3).        -   Group rp4 can be further included as a hierarchical subgroup            in another group in the same design.

The construction of the resulting hierarchical relative placementstructure is shown in FIG. 5.

Groups rp1, rp2, and rp3 are from the same design, top_design. They areincluded in group rp4, which can be further included one time intop_design.

Defining Hierarchical Groups for Instantiation

Specify that a group is a hierarchically instantiated group byspecifying hierarchy plus an instance name with the add_to_rp_groupcommand.

Instantiating a group is a useful way to replicate relative placementinformation across multiple instances of a design and to create relativeplacement relationships between those instances. An instantiated groupcan be used multiple times and in multiple places. For example, variousembodiments use hierarchy instantiation for one or more of these cases:

1) Multiple relative placement layouts are to be used for differentinstances of a design.

2) Despite one layout, relative placement is to be specified betweeninstances of that layout or between instances and other cells andgroups.

The syntax for instantiation is the same as the syntax for inclusion butprovides the -instance switch in addition to the -hierarchy switch. The-instance switch specifies the hierarchical cell upon which toinstantiate the given hierarchical relative placement group. Theinstance is within the design of the group to which it is added and isan instance of the same design of the group being added hierarchically.

When uniquified, instantiated groups are dropped unless they arerequired for the newly uniquified group; that is, each instantiationwill go to one uniquified design.

See the syntax provided in “Syntax for Adding a Hierarchical Group” andthe options summary provided in Table 3.

Example

To instantiate the relative placement group named rp1 using ahierarchical cell instance named I1 in the relative placement groupnamed rp2, enter

psyn_shell-xg-t> add_to_rp_group rp2 -hierarchy rp1  -instance I1-column 0 -row 0

The script in the example below provides a definition for hierarchicalrelative placement for instantiation. Group rp1 is in the designpair_design (shown in bold) and defines leaf cells U1 and U2 as thegroup. Group rp2 is in the design mid_design (shown in bold) andinstantiates three instances of group rp1 from pair_design, named I1,I2, and I3. Each instance is defined as a subgroup plus an instance nameand each is treated as a leaf cell.

Example Definition for Hierarchical Relative Placement for Instantiation

create_rp_group rp1 -design pair_design -columns 2 -rows 1 add_to_rp_group rp1 -leaf U1 -column 0 -row 0  add_to_rp_group rp1-leaf U2 -column 1 -row 0 create rp_group rp2 -design mid_design-columns 1 -rows 3  add_to_rp_group rp2 -hier rp1 -instance I1 -column 0-row 0  add_to_rp_group rp2 -hier rp1 -instance I2 -column 0 -row 1 add_to_rp_group rp2 -hier rp1 -instance I3 -column 0 -row 2

In the above example,

-   -   Instances I1, I2, and I3 are hierarchical cells instantiating        the design pair_design.    -   Groups rp1 is defined as having two columns and one row and        contains leaf cells U1 and U2.    -   Group rp2, in which group rp1 is instantiated three times, is        defined as having one column and three rows. Each instantiated        group is defined as a hierarchical subgroup containing a named        instance.    -   Group rp2 is treated as a leaf cell, and can be used multiple        times if it is further instantiated.

The construction of the resulting hierarchical relative placement blockis shown in FIG. 6.

Group rp1 belongs to the design pair_design. It is instantiated threetimes in group rp2, which can be further instantiated in differentdesigns.

Ungrouping Hierarchical Relative Placement

The ungroup command changes hierarchical relative placement structure.

After using ungroup, hierarchical relative placement instantiation isconverted to hierarchical relative placement inclusion because thedesign is flattened and all the groups are now of the same design.Instantiation of hierarchical modules no longer exists.

Relative placement groups affected by an ungroup command are renamed toshow the path to the group before flattening followed by a slash (/) andthe original group name. If this results in a name collision, a numberedsuffix is added to create a unique name. For example, rp2 rp1(I3) 0 2becomes rp2 I3/rp1 0 2 after ungrouping.

Using the hierarchical block shown in FIG. 6, the relative placementdefinition is now as shown in the example below. After ungroup -flatten-all, the resulting ungrouped hierarchical placement block is as shownin FIG. 7.

Example Hierarchical Relative Placement Changed by the Ungroup Command

create_rp_group I1/rp1 -design mid_design -columns 2 -rows 1  I1/rp1-leaf I1/U1 -column 0 -row 0  I1/rp1 -leaf I1/U2 -column 1 -row 0create_rp_group I2/rp1 -design mid_design -columns 2 -rows 1  I2/rp1-leaf I2/U1 -column 0 -row 0  I2/rp1 -leaf I2/U2 -column 1 -row 0create_rp_group I3/rp1 -design mid_design -columns 2 -rows 1  I3/rp1-leaf I3/U1 -column 0 -row 0  I3/rp1 -leaf I3/U2 -column 1 -row 0create_rp_group rp2 -design mid_design -columns 1 -rows 3  rp2-hierarchy I1/rp1 -column 0 -row 0  rp2 -hierarchy I2/rp1 -column 0 -row1  rp2 -hierarchy I3/rp1 -column 0 -row 2

Uniquifying Hierarchical Relative Placement

The uniquify command can change each instantiation of hierarchicalrelative placement structure.

For example, uniquifying

group grp_top top 1 2  hier grp_ripple(U1) 0 0  hier grp_ripple(U2) 0 1group grp_ripple ripple  [ ... ]

results in

group grp_top top 1 2  hier grp_ripple_1(U1) 0 0  hier grp_ripple_2(U2)0 1 group grp_ripple_1 ripple_1 group grp_ripple_2 ripple_2

Adding Keepouts

Hard keepouts can be specified within relative placement blocks. To dothis, use the add_to_rp_group command with its -keepout switch.

When defining keepouts, the one or more of the following points canapply:

-   -   Keepouts are not objects. A name is to be provided for        reference. IN other embodiments, object keepouts are created.    -   The width and height of a keepout can be specified.        -   The unit of width for a keepout is the number of placement            sites. If the width is not specified, the default width is            the width of the widest cell in that column.        -   The unit of height for a keepout is one row. If the height            is not specified, the default height is the height of the            tallest cell in that row.

See the syntax provided in “Syntax for Adding a Keepout” and the optionssummary provided in Table 3.

Example

To create the hard keepout named gap1 shown in FIG. 8, enter

-   -   psyn_shell-xg-t>add_to_rp_group misc    -   keepout gap1    -   column 0 -row 2    -   width 15 -height 1

FIG. 8 shows a relative placement block containing keepouts 800 (namedgap1 . . . gap5 in this example). The input to define the keepouts 800is provided in the example below, following the figure.

The script in the example below provides the definition for the relativeplacement block containing keepouts shown in FIG. 8.

Example Definition for Relative Placement Input for Defining Keepouts

create_rp_group misc -design top -columns 5 -rows 4  add_to_rp_groupmisc -leaf U1 -column 0 -row 0  add_to_rp_group misc -leaf U3 -column 0-row 1  add_to_rp_group misc -keepout gap1 -column 0 -row 2 -width 15  -height 1  add_to_rp_group misc -leaf U4 -column 0 -row 3 add_to_rp_group misc -leaf U2 -column 1 -row 0  add_to_rp_group misc-keepout gap2 -column 1 -row 1 -width 15   -height 1  add_to_rp_groupmisc -keepout gap3 -column 2 -row 1 -width 10   -height 1 add_to_rp_group misc -leaf U5 -column 3 -row 0  add_to_rp_group misc-leaf U6 -column 3 -row 1  add_to_rp_group misc -leaf U7 -column 3 -row2  add_to_rp_group misc -keepout gap4 -column 3 -row 2  -width 5 -height1  add_to_rp_group misc -keepout gap5 -column 3 -row 3 -width 20  -height 1  add_to_rp_group misc -leaf U8 -column 4 -row 0 add_to_rp_group misc -leaf U9 -column 4 -row 1

Creating Relative Placement Structures Containing Multiple Column or RowPositions

A cell can occupy multiple column positions or multiple row positions,which is known as straddling. To define straddling, use the inclusionhierarchical relative placement syntax (see “Defining HierarchicalGroups for Inclusion”). When a group is an included group, it can beused once in the design in which it is defined. However, the new groupin which it is included can be included or instantiated in anothergroup.

FIG. 9 shows a relative placement group in which cells straddle columns(instance U2 901) and rows (instance U7 902).

The script in the example below provides the definition for the relativeplacement block shown in FIG. 9.

To construct the hierarchy needed for straddling, the leaf cell groupsare defined for rp1, rp2 (the cell that straddles columns 0 and 1), andrp3, then define group rp4 to contain groups rp1, rp2, and rp3. Finally,rp5 is defined to contain group rp4 and leaf cell U7 (the cell thatstraddles rows 0, 1, and 2).

Example Definition for Relative Placement Input for HierarchicalPlacement with Straddling

create_rp_group rp1 -design top -columns 2 -rows 1  add_to_rp_group rp1-leaf U1 -column 0 -row 0  add_to_rp_group rp1 -leaf U4 -column 1 -row 0create_rp_group rp2 -design top -columns 1 -rows 1  add_to_rp_group rp2-leaf U2 -column 0 -row 0 create_rp_group rp3 -design top -columns 2-rows 1  add_to_rp_group rp3 -leaf U3 -column 0 -row 0  add_to_rp_grouprp3 -leaf U6 -column 1 -row 0 create_rp_group rp4 -design top -columns 1-rows 3  add_to_rp_group rp4 -hier rp1 -column 0 -row 0  add_to_rp_grouprp4 -hier rp2 -column 0 -row 1  add_to_rp_group rp4 -hier rp3 -column 0-row 2 create_rp_group rp5 -design top -columns 2 -rows 1 add_to_rp_group rp5 -hier rp4 -column 0 -row 0  add_to_rp_group rp5-leaf U7 -column 1 -row 0

FIG. 10 shows the construction of the hierarchy defined in the exampleabove.

Specifying Orientation for Leaf Cells

By default, the placement, routing, and optimization tool doesorientation optimization (automatic orientation) for cells in relativeplacement groups but orientation can be specified for cells on aper-cell basis, use a mix of user-specified orientation and automaticorientation, or disable orientation on cells in relative placementgroups. You cannot specify orientation for a group. In some embodiments,specifying orientation for a group specifies that orientation for allcells of the group.

If an orientation is not specified for a cell, by default, the tool useseither orientation optimization or the default orientation for the cell.Orientation optimization can flip a cell from its default orientation toimprove wire length.

To specify orientation for leaf cells, use the add_to_rp_group commandwith its -orientation switch and the syntax for defining a leaf cell. Inaddition, direct the placement, routing, and optimization tool is to bedirected regarding orientation optimization.

When specifying orientation, one or more of the following points canapply:

-   -   Orientation specified has precedence over orientation        optimization and default orientation.    -   If an orientation that is not valid is specified, that        orientation is ignored and a valid orientation is used.    -   Specifying both pin alignment and orientation in the same        invocation might be contradictory. Although every attempt is        made to honor such a request, honoring both might not be        possible. In this case, the orientation specification takes        precedence over the pin alignment specification.    -   If orientation is not specified for a cell and automatic        orientation is done, pin alignment is honored.

The syntax is

-   -   add_to_rp_group group_list -leaf instance_name -column        col_number -row row_number -orientation direction

For syntax details, see “Adding Items to a Group”.

Directing Orientation Optimization

Orientation optimization can flip a cell to improve relative placementwire length, thereby improving QoR for the design. Orientationoptimization is enabled by default.

The physopt_rp_enable_orient_opt variable controls whether orientationoptimization is enabled (default true). Orientation optimization isenabled or disabled according to whether to specify the orientation forsome cells or disable orientation optimization.

Specifying a Mix of User-Specified Orientation and Automatic Orientation

Orientation can be specified for some cells in a group and automaticorientation allowed for the other cells. To do this, ensure that thephysopt_rp_enable_orient_opt variable is set to true (the default).

This ensures that orientations specified are respected and automaticorientation is done for the other cells.

Disabling Orientation Optimization

Orientation optimization can be disabled by settingphysopt_rp_enable_orient_opt to false (default is true), for example,when pin alignment is to have precedence.

When this variable is set to false, the specified orientation isrespected if the orientation is valid. If no user-specified orientationexists, a default valid orientation is chosen.

Specifying Orientation and Pin Alignment

Both orientation and pin alignment can be specified in the sameinvocation but doing this might be contradictory.

When used with pin alignment, in various embodiments the priorities fororientation are as follows, in this order:

1. User-Specified Orientation

When used with pin alignment, orientation has precedence.

2. Orientation Optimization

If orientation is not specified for a cell and orientation optimizationis done, pin alignment is honored.

3. Default Orientation

When used with pin alignment, pin alignment has precedence.

Other embodiments remove, add to, or rearrange the above priorities.

FIG. 11 shows orientation optimization used with pin alignment, inparticular pin A 1100. In such a case, both orientation and pinalignment are honored. (Not all leaf cells listed in the example thatfollows the figure are shown in the figure.)

The example below provides the definition for the relative placementshown in FIG. 11.

Example Orientation Optimization Used with Pin Alignment

create_rp_group misc1 -design block1 -columns 3 -rows 10 -pin_alignmentA  add-to_rp_group misc1 -leaf I3 -column 0 -row 0 -orientation N add-to_rp_group misc1 -leaf I4 -column 0 -row 1 -orientation FN add-to_rp_group misc1 -leaf I5 -column 0 -row 2 -orientation N add-to_rp_group misc1 -leaf I6 -column 0 -row 3  add-to_rp_group misc1-leaf I7 -column 0 -row 4  add-to_rp_group misc1 -leaf I8 -column 0 -row5

Writing Relative Placement Information to a Script File

Specified relative placement groups can be written to a named file,creating a Tcl-format script for recreating relative placement groupsand their items on the same design. To do this, use the write_rp_groupcommand.

The command returns a collection handle (identifier) of relativeplacement groups written out. If no objects were written, the emptystring is returned.

To use the write_rp_group command, enter psyn_shell-xg-t>write_rp_group[options]

To do this Use this List the groups to write to the script. (vs. using-all). group_list Write all the relative placement groups to the script.-all (vs. using a group list or -hierarchy). Write all the relativeplacement groups within the -hierarchy hierarchy of the relativeplacement groups. By omitting this option, subgroups are not written.Disable line splitting when information exceeds -nosplit the columnwidth. Write the script to the specified file. By omitting -output thisoption, the information is written to the screen.

Example

To save all the relative placement groups to disk, remove theinformation from the design, then recreate the information on thedesign, enter

psyn_shell-xg-t> find rp_group {mul::grp_mul ripple::grp_rippleexample3::top_group} psyn_shell-xg-t> write_rp_group -all -outputmy_groups.tcl 1 psyn_shell-xg-t> remove_rp_group -all -quiet 1psyn_shell-xg-t> find rp_group Error: Can't find objects matching ’*’.(UID-109) psyn_shell-xg-t> source my_groups.tcl {example3::top_group}psyn_shell-xg-t> find rp_group {example3::top_group ripple::grp_ripplemul::grp_mul}

Aligning Relative Placement by Pins

Columns can be aligned by pins instead of by the lower-left corner (thedefault). This capability increases the probability of straight routesand can result in less congestion, lower power, and lower routingresources by eliminating vias.

To align a group by pins, use the create_rp_group command with its-pin_align_name switch.

When aligning by pins, one or more of the following points can apply:

-   -   When specifying a pin name, the tool determines the location for        that pin in cells in the column, then aligns the column based on        the pin locations.    -   If cells in a column do not have the pin specified, the column        is aligned as follows:        -   If some cells in a column do not have the pins specified,            those cells are aligned with a default position (e.g., the            lower-left corner) and an information message appears.        -   If no cells in a column have the pins specified, the cells            are aligned with a default position (e.g., the lower-left            corner) and a warning appears.    -   Aligning by pins can result in empty space between columns.    -   Both pin alignment and orientation can be specified in the same        invocation but doing this might be contradictory. Although every        attempt is made to honor such a request, honoring both might not        be possible. In this case, the orientation specification takes        precedence over the pin alignment specification.    -   The widest cell in the column determines the width of the        column.

FIG. 12 shows a relative placement group aligned by pins.

The script in the example below defines the relative placement groupshown in FIG. 12, which is aligned by pin A 1201.

Example Definition for Relative Placement Group Aligned by Pins

create_rp_group rp1 -design pair_design -columns 1 -rows 4 -pin_align_name A  add_to_rp_group rp1 -leaf U1 -column 0 -row 0 add_to_rp_group rp1 -leaf U2 -column 0 -row 1  add_to_rp_group rp1-leaf U3 -column 0 -row 2  add_to_rp_group rp1 -leaf U4 -column 0 -row 3

A column can be aligned within a placement group by a specified pin andalign cells within the column by a different pin as shown in FIG. 13,with pin A 1301 and pin B 1302. The alignment pin name specified forparticular cells in the column overrides the alignment pin namespecified for the group.

A set of cells can be specified to align over specified pins. Forexample, pins A and B can be aligned in a group by specifying adifferent pin alignment name for some cells.

The script in the example below defines the relative placement groupshown in FIG. 13. In the example, the group misc1 is aligned by pin Aand instances I5 and I6 within the group are aligned by pin B,overriding the group pin alignment name A for those instances.

Example Definition to Align a Group and Leaf Cells by Pins

create_rp_group misc1 -design block1 -columns 3 -rows 10 -pin_align_nameA  add_to_rp_group misc1 -leaf I3 -column 0 -row 0  add_to_rp_groupmisc1 -leaf I4 -column 0 -row 1  add_to_rp_group misc1 -leaf I5 -column0 -row 2 -pin_align_name B  add_to_rp_group misc1 -leaf I6 -column 0-row 3 -pin_align_name B  add_to_rp_group misc1 -leaf I7 -column 0 -row4  add_to_rp_group misc1 -leaf I8 -column 0 -row 5

FIG. 14 shows a relative placement block that contains a group alignedby pins, in particular pin CLK 1401—the column named bank1 (col 0). Itis included in the group named final. Group final can also be usedfurther for instantiation or inclusion in another group.

The script in the example below provides the definition for the relativeplacement block shown in FIG. 14.

Example Definition for Hierarchical Relative Placement Block with ColumnAligned by Pins

create_rp_group bank1 -design top -columns 1 -rows 4 -pin_name clk add_to_rp_group bank1 -leaf U1 -column 0 -row 0  add_to_rp_group bank1-leaf U2 -column 0 -row 1  add_to_rp_group bank1 -leaf U3 -column 0 -row2  add_to_rp_group bank1 -leaf U4 -column 0 -row 3 create_rp_group bank2-design top -columns 1 -rows 2  add_to_rp_group bank2 -leaf U5 -column0-row 0  add_to_rp_group bank2 -leaf U6 -column 0-row 1 create rp_groupbank4 -design top -columns 1 -rows 4  add_to_rp_group bank4 -leaf U19-column 0 -row 0  ...  add_to_rp_group bank4 -leaf U22 -column 0 -row 3create_rp_group comb1 -design top -columns 3 -rows 1  add_to_rp_groupcomb1 -leaf U7 -column 0 -row 0  add_to_rp_group comb1 -leaf U8 -column1 -row 0  add_to_rp_group comb1 -leaf U9 -column 2 -row 0create_rp_group comb2 -design top -columns 3 -rows 1  add_to_rp_groupcomb2 -leaf U10 -column 0 -row 0  add_to_rp_group comb2 -leaf U11-column 1 -row 0  add_to_rp_group comb2 -leaf U12 -column 2 -row 0create_rp_group comb3 -design top -columns 3 -rows 1  add_to_rp_groupcomb3 -leaf U13 -column 0 -row 0  add_to_rp_group comb3 -leaf U14-column 1 -row 0  add_to_rp_group comb3 -leaf U15 -column 2 -row 0create_rp_group comb4 -design top -columns 3 -rows 1  add_to_rp_groupcomb4 -leaf U16 -column 0 -row 0  add_to_rp_group comb4 -leaf U17-column 1 -row 0  add_to_rp_group comb4 -leaf U18 -column 2 -row 0create_rp_group bank3 -design top -columns 1 -rows 4  add_to_rp_groupbank3 -hierarchy comb1 -column 0 -row 0  add_to_rp_group bank3-hierarchy comb2 -column 0 -row 1  ... create_rp_group final -design top-columns 4 -rows 1  add_to_rp_group final -hierarchy bank1 -column 0-row 0  add_to_rp_group final -hierarchy bank2 -column 1 -row 0 add_to_rp_group final -hierarchy bank3 -column 2 -row 0 add_to_rp_group final -hierarchy bank4 -column 3 -row 0

Anchoring Relative Placement Blocks at a Specified Location

A single relative placement block can be anchored at a locationspecified. Anchoring allows controlled placement of the relativeplacement block with respect to other relative placement blocks, macros,or to the edges and origin of the core area.

To anchor a relative placement group, use the create_rp_group commandwith its -x_offset and -y_offset switches.

When specifying an anchor point, one or more of the following points canapply:

-   -   Provide anchor points for top level groups. Anchor points are        allowed at the top level.    -   Both the x- and y-coordinates or either the x- or y-coordinate        can be specified. Specifying one coordinate as fixed allows the        unspecified coordinate to slide. The offset is an integer, in        microns, relative to the chip's origin.    -   If an anchor point outside the design boundary is specified,        relative placement alignment for the block fails, a warning        appears, and the cells are clustered inside the boundary.    -   If an anchor point is specified for a group that is not a        top-level group or that causes placement that is not valid, a        warning appears and relative placement continues.

FIG. 15 shows a relative placement block anchored at both thex-coordinate and the y-coordinate.

The script in the example below provides the definition for anchoringrelative placement block misc1 in block 1 at both x-coordinate 100 andy-coordinate 100. (In both the figure and the example, not all rows areshown.)

Example Definition for Anchoring a Group Using Two Coordinates

create_rp_group misc1 -design block1 -columns 3 -rows 10  -x_offset 100-y_offset 100 add_to_rp_group misc1 -leaf I30 -column 0 -row 0add_to_rp_group misc1 -leaf I31 -column 0 -row 1 add_to_rp_group misc1-leaf I32 -column 0 -row 2 add_to_rp_group misc1 -leaf I33 -column 0-row 3 add_to_rp_group misc1 -leaf I34 -column 0 -row 4 add_to_rp_groupmisc1 -leaf I35 -column 0 -row 5 ...

FIG. 16 shows 12 relative placement blocks aligned and anchoredvertically at four coordinates. Blocks 1, 2, and 3 1601 have -x_offset100. Blocks 4, 5, and 6 1602 have -x_offset 200. Blocks 7, 8, and 9 1603have -x_offset 300. Blocks 10, 11, and 12 1604 have -x_offset 400.

The script in the example below defines the locations of the 12vertically aligned and anchored relative placement blocks shown in FIG.16. For brevity, not every group is listed in the example.

Example Definitions for Locations of Vertically Aligned and AnchoredBlocks

create_rp_group block1 -design misc1 -columns 3 -rows 10 -x_offset 100create_rp_group block2 -design misc1 -columns 3 -rows 10 -x_offset 100create_rp_group block3 -design misc1 -columns 3 -rows 10 -x_offset 100create_rp_group block4 -design misc1 -columns 3 -rows 10 -x_offset 200create_rp_group block5 -design misc1 -columns 3 -rows 10 -x_offset 200create_rp_group block6 -design misc1 -columns 3 -rows 10 -x_offset 200... create_rp_group block12 -design misc1 -columns 3 -rows 10 -x_offset400

Using Compression to Remove Empty Space in a Relative Placement Group

By default, construction for relative placement aligns cells from theirbottom-left corner. Compression removes empty space in rows to create amore compact structure. The compressed columns are no longer aligned andutilization is higher in the area of the compressed cells.

If compression is needed, use hierarchical relative placement toconstruct the pattern, using the syntax for hierarchical inclusion.

FIG. 17 shows the same cells aligned without compression 1701 and withcompression 1702. The cells are bottom-left aligned.

Alternatively, compression can be accomplished by using bit-stackplacement. Set the variable physopt_bit_stacked_placement to true (thedefault is false). Setting this variable to true causes the empty spaceto be removed, compressing the group as shown in FIG. 17. The columnsare no longer aligned and utilization is higher.

Relative Placement in a Design Containing Obstructions

During placement, relative placement groups can avoid placement keepouts(obstructions) that are defined in the PDEF file or created by a theplacement, routing, and optimization tool keepout command(create_placement_keepout, create_wiring_keepout). A relative placementgroup can be broken into pieces that straddle obstructions.

FIG. 18 shows the placement of relative placement cells in a designcontaining keepouts 1801 that were either defined in the PDEF file orcreated by a the placement, routing, and optimization tool keepoutcommand. Rows 0 and 2 and column 5 are placed to avoid the keepouts butrelative placement alignment is maintained.

Converting Rp_Reader Syntax to Tcl Syntax

Use the following transformations to convert existing rp_reader textfiles to Tcl syntax to use within the placement, routing, andoptimization tool:

-   -   Change the keyword group to the create_rp_group command    -   Insert the add_to_rp_group command before each item in a group    -   Change the keyword keepout to -keepout    -   Change the keyword hierarchy to -hierarchy    -   Change the keyword leaf to -leaf    -   Insert the -design switch before the design name when creating a        relative placement group    -   Insert the -column and -row switches in front of those values    -   Insert the -width and -height switches in front of those values

Alternatively, the following command can be used that runs a script todo the conversion:

-   -   rp_reader dbfile out.tcl -tcl_export

The following tables show the rp_reader file format elements for groups,leaf cells, hierarchy groups, and keepouts.

Group

group misc top 9 10 keyword group design number of number name namecolumns of rows

Leaf Cell

custom leaf U0 0 0 group keyword instance column row name name positionposition

Hierarchy Group for Inclusion

rp4 hier rp1 0 0 group keyword subgroup column row name name positionposition

Hierarchy Group for Instantiation

rp2 hier rp1 (U3) 0 0 group keyword subgroup instance column row namename name position position

Keepout

misc keep gap4 3 2 5 1 group keyword keepout column row width heightname name position position

FIG. 19 is a simplified block diagram of a computer system 1910 suitablefor use with embodiments of the technology. Computer system 1910typically includes at least one processor 1914 which communicates with anumber of peripheral devices via bus subsystem 1912. These peripheraldevices may include a storage subsystem 1924, comprising a memorysubsystem 1926 and a file storage subsystem 1928, user interface inputdevices 1922, user interface output devices 1920, and a networkinterface subsystem 1916. The input and output devices allow userinteraction with computer system 1910. Network interface subsystem 1916provides an interface to outside networks, including an interface tocommunication network 1918, and is coupled via communication network1918 to corresponding interface devices in other computer systems.Communication network 1918 may comprise many interconnected computersystems and communication links. These communication links may bewireline links, optical links, wireless links, or any other mechanismsfor communication of information. While in one embodiment, communicationnetwork 1918 is the Internet, in other embodiments, communicationnetwork 1918 may be any suitable computer network.

User interface input devices 1922 may include a keyboard, pointingdevices such as a mouse, trackball, touchpad, or graphics tablet, ascanner, a touchscreen incorporated into the display, audio inputdevices such as voice recognition systems, microphones, and other typesof input devices. In general, use of the term “input device” is intendedto include all possible types of devices and ways to input informationinto computer system 1910 or onto computer network 1918.

User interface output devices 1920 may include a display subsystem, aprinter, a fax machine, or non-visual displays such as audio outputdevices. The display subsystem may include a cathode ray tube (CRT), aflat-panel device such as a liquid crystal display (LCD), a projectiondevice, or some other mechanism for creating a visible image. Thedisplay subsystem may also provide non-visual display such as via audiooutput devices. In general, use of the term “output device” is intendedto include all possible types of devices and ways to output informationfrom computer system 1910 to the user or to another machine or computersystem.

Storage subsystem 1924 stores the basic programming and data constructsthat provide the functionality of certain embodiments. For example, thevarious modules implementing the functionality of certain embodimentsmay be stored in storage subsystem 1924. These software modules aregenerally executed by processor 1914.

Memory subsystem 1926 typically includes a number of memories includinga main random access memory (RAM) 1930 for storage of instructions anddata during program execution and a read only memory (ROM) 1932 in whichfixed instructions are stored. File storage subsystem 1928 providespersistent storage for program and data files, and may include a harddisk drive, a floppy disk drive along with associated removable media, aCD-ROM drive, an optical drive, or removable media cartridges. Thedatabases and modules implementing the functionality of certainembodiments may be stored by file storage subsystem 1928.

Bus subsystem 1912 provides a mechanism for letting the variouscomponents and subsystems of computer system 1910 communicate with eachother as intended. Although bus subsystem 1912 is shown schematically asa single bus, alternative embodiments of the bus subsystem may usemultiple busses.

Computer readable medium 1940 can be a medium associated with filestorage subsystem 1928, and/or with network interface 1916. The computerreadable medium can be a hard disk, a floppy disk, a CD-ROM, an opticalmedium or removable media cartridge. The computer readable medium 1940is shown storing a circuit design 1980 created with the describedtechnology. Also shown is a circuit 1990 created with the describedtechnology.

Computer system 1910 itself can be of varying types including a personalcomputer, a portable computer, a workstation, a computer terminal, anetwork computer, a television, a mainframe, or any other dataprocessing system or user device. Due to the ever-changing nature ofcomputers and networks, the description of computer system 1910 depictedin FIG. 19 is intended only as a specific example for purposes ofillustrating the preferred embodiments. Many other configurations ofcomputer system 1910 are possible having more or less components thanthe computer system depicted in FIG. 19.

Automatic Relative Placement Rules at RTL-Level

Relative placement rules can be generated the RTL-level. RP takesadvantage of inherent structure at the RTL-level of behavioraldescription, in addition to at the netlist cell level.

When relative placement rules can be generated at the RTL-level, the RPconstraints are not required to be specific to the netlist, as the RPconstraints are not required to be tied to the instance names of thecells in the netlist. Thus, every time a new netlist is synthesized, andthe netlist has different instance names from a previously synthesizednetlist despite functionally identical designs, the RP constraints arenot invalidated and need not be re-written to reference new cellinstance names. This process is easily repeatable. RP constraints at theRTL-level are portable even when a new netlist is created throughadditional synthesis.

When relative placement rules can be generated at the RTL-level, thencorresponding RP constraints at the cell instance level do not have tobe written. This saves time, because writing RP constraints at the cellinstance level is a very detailed and tedious task.

This technology focuses on RP specification on high level RTL constructsand subsequent automatic cell level RP generation from thespecification. It frees the user from writing the cell level RPconstraints and rewriting them when the design is re-synthesized. Thesolution also has the full implementation flow support from RTLsynthesis to place and route.

In one example design flow, an RTL hardware description is processed bya synthesis tool such as Synopsys Design Compiler into a netlist. Thesynthesis tool automatically creates RTL-level RP rules. When thesynthesis tool creates the netlist, the corresponding netlist-level RPrules are automatically created based on the RTL-level RP rules. A placeand route and optimization tool such as Synopsys IC Compiler thengenerates the placed, routed, and optimized design from the netlist andthe netlist-level RP rules. Generated netlist-level RP rules areautomatically updated reflecting name changes on composing cells whendesign hierarchy is being ungrouped, uniquified, change_named orchange_linked.

The RP constraints are propagated to synthesis and place and route toolssuch as DCT and ICC.

Automatic Relative Placement for Clock Trees

The application of automatic relative placement to clock trees helps toprevent overdesign from adding too many levels of buffers to the clocknetwork, and helps to prevent oversizing the drivers of the buffers.Because overdesign is reduced, the clock tree can handle higher fanoutsfrom the clock drivers for relative placement.

FIG. 20 is a flow diagram illustrating an exemplary process of theplacement, routing, and optimization tool with clock tree synthesisclustering being performed during placement.

The process flow of FIG. 20 is similar to the process of FIG. 2, andadds clock tree synthesis clustering during placement. In 2002, coarseplacement is performed on the netlist. In 2004, during placement, clocktree synthesis is applied to cluster leaf-level clock flip-flops.

In 2006, during placement, relative placement groups are automaticallydefined based on the results from the clock tree synthesis clustering ofleaf-level clock flip-flops. There are four cases shown—the fourcombinations of netlists with and integrated clock gating, and clocktree buffers added or not added. In 2006A, the netlist is withoutintegrated clock gating, and buffers are added before each of therelative placement groups. Clock buffers amplify clock signals degradedby interconnect impedance, and isolate clock nets from upstream loadimpedance of the clock tree. Because buffers are added, the nets arebroken between the clock and the relative placement groups. In 2006B,the netlist is without integrated clock gating, and buffers are notadded before each of the relative placement groups. Because buffers arenot added, the nets are not broken between the clock and the relativeplacement groups. In 2006C, the netlist is with integrated clock gating,and integrated clock gating is added in each of the relative placementgroups. In 2006D, the netlist is with integrated clock gating, andintegrated clock gating is not added in each of the relative placementgroups, such that the integrated clock gating is freely movable by theplacer.

In 2009, incremental placement of relative placement blocks, anddetailed placement of non-relative placement cells, is performed. In2018, physical optimization of cells is performed. In 2020, relativeplacement constraints are applied. In 2030, clock tree synthesis isapplied to build the complete clock tree(s).

FIG. 21 is a flow diagram illustrating another exemplary process of theplacement, routing, and optimization tool with clock tree synthesisclustering being performed during placement.

In 2101, the annotated netlist is preprocessed for relative placementannotation. Data structures are created to carry relative placementinformation. In 2102, clock drivers such as integrated clock gating arescanned in the netlist (or user input received for a given clock net,clock pin, or clock name), etc., and clock sinks for a given clockobject (integrated clock gating, clock pin, clock name, clock pin, etc.)are collected for post-initial placement. In 2104, user clock treesynthesis constraints are read in, such as clock tree references. Clocktree constraints are specified early for relative placement, rather thanwaiting for post-placement full clock tree synthesis.

Clock tree constraints are listed as follows, with a description ofconstraint. None, one, or more of the clock tree constraints can be ineffect. The scope of each constraint can be global or per clock. Variousconstraints specify: clock trees that the options apply to; maximumcapacitance constraint; maximum fanout constraint; maximum transitiontime constraint; maximum skew goal; minimum insertion delay goal;maximum number of clock tree levels; preferred layers for clock treerouting; nondefault routing rule used for clock tree routing; whetherdefault routing rules are used for the clock tree levels closest to theclock sink; whether to enable/disable boundary cell insertion; whetherto cluster based on minimum wire length; whether to cluster based onon-chip variation; whether to enable/disable logic-level balancing;whether to enable/disable buffer relocation during optimization; whetherto enable/disable buffer sizing during optimization; whether toenable/disable delay insertion during optimization; whether toenable/disable gate relocation during optimization; whether toenable/disable gate sizing during optimization; the clock configurationfile to read; and the name of the clock configuration file that iswritten after clock tree synthesis.

In 2106, clock tree synthesis clustering is performed. Prior toclustering, preexisting relative placement cells, invalid relativeplacement objects etc. are filtered out of list of flip-flops to beclustered. Responsive to calling the compile clock tree clusteringalgorithm, the clock tree synthesis clusters are returned. Multiplecluster handling includes checking all elements in a cluster to verifythat they belong to the same hierarchy. If all elements do not belong tothe same hierarchy, then a relative placement group is not created fromthe cluster.

In 2108, clusters from the clock tree synthesis clustering areprocessed, and relative placement groups are formed form the clusters;geometric compression is performed; and the shapes of relative placementgroups is automatically determined. Example geometric compressiondetails follow. Existing RP elements are filtered from collection.Sorting is performed based on X location of each cell (alternatively, Ylocation). The number of rows and columns is automatically determinedbased on the total number of elements. An example of this determinationfollows: MaxRows=f(RPShape,userinput); FormFactor=f(Macflops,MaxRows);NumRows=f(FormFactor,NumFlops); NumCols=f(Numflops,clockdriver,NumRows).Finally, elements are sorted within each column based on Y location ofeach cell (alternatively, X location).

In 2110, integrated clock gating is added, or clock drivers are added tothe clusters from clock tree synthesis clustering. Nets are broken asneeded to make such additions of the integrated clock gating and theclock drivers. For example, if a buffer needs to be inserted, then thenet is broken, the buffer is created, and the buffer is connected. Inanother example, clock drivers of correct size are chosen and inserted.In 2112, relative placement options are set for each relative placementgroup. Examples of relative placement options are bottom left alignment(alternatively bottom right, top right, top left), pin alignment (suchas clock pin), orientation (such as to minimize wire crossing within arelative placement group), X offset and/or Y offset (in case of anchoredlocations, anchor of RP group derived based on initial locations offlip-flops and ICG, and center of gravity used to determine X and/or Yoffsets), routing and optimization options, move values for RP groups(how much a group can move while doing overlap removal, etc. In 2114,the coarse placement is refined. In the next pass of coarse placement,the relative placement groups are placed. Relative placement groups arelegalized respecting all automatically generated or manually specifiedrelative placement constraints. In 2116, optimization occurs. In 2118,size is fixed/marked for the relative placement groups for clock treesynthesis. In 2120, the full clock tree synthesis is performed.

While the present invention is disclosed by reference to the embodimentsand examples detailed above, it is to be understood that these examplesare intended in an illustrative rather than in a limiting sense. It iscontemplated that modifications and combinations will readily occur tothose skilled in the art, which modifications and combinations will bewithin the spirit of the invention and the scope of the followingclaims.

What is claimed is:
 1. A method of circuit design executed by a computerimplemented place and route tool, comprising: processing a net listusing the tool to determine relative placement groups identified by therelative placement information in the netlist, and to determine clocktree sinks in the netlist; performing clock tree synthesis using thetool to add one or more elements to the input netlist from a group ofelements including buffers, integrated clock gating, and clock drivers,to clock network flip-flops coupled to the clock tree sinks in clockclusters, and allocating the clock network flip-flops and added elementsin the clock clusters to relative placement groups; performing aninitial placement using the tool to specify physical locations of therelative placement groups, including relative placement groups to whichthe clock network flip-flops and added elements in the clock clustershave been allocated; and performing an incremental placement using thetool based on the specified physical locations of the relative placementgroups determined by the initial placement.
 2. The method of claim 1,further including: adding the clock network flip-flops to the netlistbefore performing the clock tree synthesis.
 3. The method of claim 1,wherein the allocating of the clock network flip-flops and addedelements to relative placement groups, includes changing a preexistingrelative placement group.
 4. The method of claim 1, wherein the clocknetwork flip-flops are leaf nodes of a clock tree of the circuit design.5. The method of claim 1, including performing a first placement usingthe first relative placement groups determined before said performingclock tree synthesis, and using information from the first placement inthe clock tree synthesis.
 6. A computer program product including atangible non-transitory computer readable medium with computer readableinstructions executable by a computer system, the computer readableinstructions comprising: instructions processing a net list to determinerelative placement groups and clock tree sinks; instructions performingclock tree synthesis to add one or more elements to the input netlistfrom a group of elements including buffers, integrated clock gating, andclock drivers, to clock network flip-flops coupled to the clock treesinks in clock clusters, and allocating the clock network flip-flops andadded elements in the clock clusters to the relative placement groups;instructions performing an initial placement to specify physicallocations of the relative placement groups, including relative placementgroups to which the clock network flip-flops and added elements in theclock clusters have been allocated; and instructions performing anincremental placement using the physical locations of the relativeplacement groups determined by the initial placement.
 7. The computerprogram product of claim 6, further including: instructions adding clocknetwork flip-flops to the netlist before performing the clock treesynthesis.
 8. The computer program product of claim 6, furtherincluding: the instructions allocating of the clock network flip-flopsand added elements to relative placement groups, changing a preexistingrelative placement group.
 9. The computer program product of claim 6,wherein the clock network flip-flops are leaf nodes of a clock tree ofthe circuit design.
 10. The computer program product of claim 6,including instructions performing a first placement using relativeplacement groups determined before said performing clock tree synthesis,and using information from the first placement in the clock treesynthesis.